/*
 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.lang;

/**
 * Class {@code Object} is the root of the class hierarchy.
 * Every class has {@code Object} as a superclass. All objects,
 * including arrays, implement the methods of this class.
 *
 * @author unascribed
 * @see java.lang.Class
 * @since JDK1.0
 */
public class Object {

	private static native void registerNatives();

	static {
		registerNatives();
	}

	/**
	 * Returns the runtime class of this {@code Object}. The returned
	 * {@code Class} object is the object that is locked by {@code
	 * static synchronized} methods of the represented class.
	 *
	 * <p><b>The actual result type is {@code Class<? extends |X|>}
	 * where {@code |X|} is the erasure of the static type of the
	 * expression on which {@code getClass} is called.</b> For
	 * example, no cast is required in this code fragment:</p>
	 *
	 * <p>
	 * {@code Number n = 0;                             }<br>
	 * {@code Class<? extends Number> c = n.getClass(); }
	 * </p>
	 *
	 * @return The {@code Class} object that represents the runtime
	 * class of this object.
	 * @jls 15.8.2 Class Literals
	 */
	public final native Class<?> getClass();

	/**
	 * Returns a hash code value for the object. This method is
	 * supported for the benefit of hash tables such as those provided by
	 * {@link java.util.HashMap}.
	 * <p>
	 * The general contract of {@code hashCode} is:
	 * <ul>
	 * <li>Whenever it is invoked on the same object more than once during
	 *     an execution of a Java application, the {@code hashCode} method
	 *     must consistently return the same integer, provided no information
	 *     used in {@code equals} comparisons on the object is modified.
	 *     This integer need not remain consistent from one execution of an
	 *     application to another execution of the same application.
	 * <li>If two objects are equal according to the {@code equals(Object)}
	 *     method, then calling the {@code hashCode} method on each of
	 *     the two objects must produce the same integer result.
	 * <li>It is <em>not</em> required that if two objects are unequal
	 *     according to the {@link java.lang.Object#equals(java.lang.Object)}
	 *     method, then calling the {@code hashCode} method on each of the
	 *     two objects must produce distinct integer results.  However, the
	 *     programmer should be aware that producing distinct integer results
	 *     for unequal objects may improve the performance of hash tables.
	 * </ul>
	 * <p>
	 * As much as is reasonably practical, the hashCode method defined by
	 * class {@code Object} does return distinct integers for distinct
	 * objects. (This is typically implemented by converting the internal
	 * address of the object into an integer, but this implementation
	 * technique is not required by the
	 * Java&trade; programming language.)
	 *
	 * @return a hash code value for this object.
	 * @see java.lang.Object#equals(java.lang.Object)
	 * @see java.lang.System#identityHashCode
	 */
	public native int hashCode();

	/**
	 * Indicates whether some other object is "equal to" this one.
	 * <p>
	 * The {@code equals} method implements an equivalence relation
	 * on non-null object references:
	 * <ul>
	 * <li>It is <i>reflexive</i>: for any non-null reference value
	 *     {@code x}, {@code x.equals(x)} should return
	 *     {@code true}.
	 * <li>It is <i>symmetric</i>: for any non-null reference values
	 *     {@code x} and {@code y}, {@code x.equals(y)}
	 *     should return {@code true} if and only if
	 *     {@code y.equals(x)} returns {@code true}.
	 * <li>It is <i>transitive</i>: for any non-null reference values
	 *     {@code x}, {@code y}, and {@code z}, if
	 *     {@code x.equals(y)} returns {@code true} and
	 *     {@code y.equals(z)} returns {@code true}, then
	 *     {@code x.equals(z)} should return {@code true}.
	 * <li>It is <i>consistent</i>: for any non-null reference values
	 *     {@code x} and {@code y}, multiple invocations of
	 *     {@code x.equals(y)} consistently return {@code true}
	 *     or consistently return {@code false}, provided no
	 *     information used in {@code equals} comparisons on the
	 *     objects is modified.
	 * <li>For any non-null reference value {@code x},
	 *     {@code x.equals(null)} should return {@code false}.
	 * </ul>
	 * <p>
	 * The {@code equals} method for class {@code Object} implements
	 * the most discriminating possible equivalence relation on objects;
	 * that is, for any non-null reference values {@code x} and
	 * {@code y}, this method returns {@code true} if and only
	 * if {@code x} and {@code y} refer to the same object
	 * ({@code x == y} has the value {@code true}).
	 * <p>
	 * Note that it is generally necessary to override the {@code hashCode}
	 * method whenever this method is overridden, so as to maintain the
	 * general contract for the {@code hashCode} method, which states
	 * that equal objects must have equal hash codes.
	 *
	 * @param obj the reference object with which to compare.
	 * @return {@code true} if this object is the same as the obj
	 * argument; {@code false} otherwise.
	 * @see #hashCode()
	 * @see java.util.HashMap
	 */
	public boolean equals(Object obj) {
		return (this == obj);
	}

	/**
	 * Creates and returns a copy of this object.  The precise meaning
	 * of "copy" may depend on the class of the object. The general
	 * intent is that, for any object {@code x}, the expression:
	 * <blockquote>
	 * <pre>
	 * x.clone() != x</pre></blockquote>
	 * will be true, and that the expression:
	 * <blockquote>
	 * <pre>
	 * x.clone().getClass() == x.getClass()</pre></blockquote>
	 * will be {@code true}, but these are not absolute requirements.
	 * While it is typically the case that:
	 * <blockquote>
	 * <pre>
	 * x.clone().equals(x)</pre></blockquote>
	 * will be {@code true}, this is not an absolute requirement.
	 * <p>
	 * By convention, the returned object should be obtained by calling
	 * {@code super.clone}.  If a class and all of its superclasses (except
	 * {@code Object}) obey this convention, it will be the case that
	 * {@code x.clone().getClass() == x.getClass()}.
	 * <p>
	 * By convention, the object returned by this method should be independent
	 * of this object (which is being cloned).  To achieve this independence,
	 * it may be necessary to modify one or more fields of the object returned
	 * by {@code super.clone} before returning it.  Typically, this means
	 * copying any mutable objects that comprise the internal "deep structure"
	 * of the object being cloned and replacing the references to these
	 * objects with references to the copies.  If a class contains only
	 * primitive fields or references to immutable objects, then it is usually
	 * the case that no fields in the object returned by {@code super.clone}
	 * need to be modified.
	 * <p>
	 * The method {@code clone} for class {@code Object} performs a
	 * specific cloning operation. First, if the class of this object does
	 * not implement the interface {@code Cloneable}, then a
	 * {@code CloneNotSupportedException} is thrown. Note that all arrays
	 * are considered to implement the interface {@code Cloneable} and that
	 * the return type of the {@code clone} method of an array type {@code T[]}
	 * is {@code T[]} where T is any reference or primitive type.
	 * Otherwise, this method creates a new instance of the class of this
	 * object and initializes all its fields with exactly the contents of
	 * the corresponding fields of this object, as if by assignment; the
	 * contents of the fields are not themselves cloned. Thus, this method
	 * performs a "shallow copy" of this object, not a "deep copy" operation.
	 * <p>
	 * The class {@code Object} does not itself implement the interface
	 * {@code Cloneable}, so calling the {@code clone} method on an object
	 * whose class is {@code Object} will result in throwing an
	 * exception at run time.
	 *
	 * @return a clone of this instance.
	 * @throws CloneNotSupportedException if the object's class does not
	 *                                    support the {@code Cloneable} interface. Subclasses
	 *                                    that override the {@code clone} method can also
	 *                                    throw this exception to indicate that an instance cannot
	 *                                    be cloned.
	 * @see java.lang.Cloneable
	 */
	protected native Object clone() throws CloneNotSupportedException;

	/**
	 * Returns a string representation of the object. In general, the
	 * {@code toString} method returns a string that
	 * "textually represents" this object. The result should
	 * be a concise but informative representation that is easy for a
	 * person to read.
	 * It is recommended that all subclasses override this method.
	 * <p>
	 * The {@code toString} method for class {@code Object}
	 * returns a string consisting of the name of the class of which the
	 * object is an instance, the at-sign character `{@code @}', and
	 * the unsigned hexadecimal representation of the hash code of the
	 * object. In other words, this method returns a string equal to the
	 * value of:
	 * <blockquote>
	 * <pre>
	 * getClass().getName() + '@' + Integer.toHexString(hashCode())
	 * </pre></blockquote>
	 *
	 * @return a string representation of the object.
	 */
	public String toString() {
		return getClass().getName() + "@" + Integer.toHexString(hashCode());
	}

	/**
	 * Wakes up a single thread that is waiting on this object's
	 * monitor. If any threads are waiting on this object, one of them
	 * is chosen to be awakened. The choice is arbitrary and occurs at
	 * the discretion of the implementation. A thread waits on an object's
	 * monitor by calling one of the {@code wait} methods.
	 * <p>
	 * The awakened thread will not be able to proceed until the current
	 * thread relinquishes the lock on this object. The awakened thread will
	 * compete in the usual manner with any other threads that might be
	 * actively competing to synchronize on this object; for example, the
	 * awakened thread enjoys no reliable privilege or disadvantage in being
	 * the next thread to lock this object.
	 * <p>
	 * This method should only be called by a thread that is the owner
	 * of this object's monitor. A thread becomes the owner of the
	 * object's monitor in one of three ways:
	 * <ul>
	 * <li>By executing a synchronized instance method of that object.
	 * <li>By executing the body of a {@code synchronized} statement
	 *     that synchronizes on the object.
	 * <li>For objects of type {@code Class,} by executing a
	 *     synchronized static method of that class.
	 * </ul>
	 * <p>
	 * Only one thread at a time can own an object's monitor.
	 *
	 * @throws IllegalMonitorStateException if the current thread is not
	 *                                      the owner of this object's monitor.
	 * @see java.lang.Object#notifyAll()
	 * @see java.lang.Object#wait()
	 */
	public final native void notify();

	/**
	 * Wakes up all threads that are waiting on this object's monitor. A
	 * thread waits on an object's monitor by calling one of the
	 * {@code wait} methods.
	 * <p>
	 * The awakened threads will not be able to proceed until the current
	 * thread relinquishes the lock on this object. The awakened threads
	 * will compete in the usual manner with any other threads that might
	 * be actively competing to synchronize on this object; for example,
	 * the awakened threads enjoy no reliable privilege or disadvantage in
	 * being the next thread to lock this object.
	 * <p>
	 * This method should only be called by a thread that is the owner
	 * of this object's monitor. See the {@code notify} method for a
	 * description of the ways in which a thread can become the owner of
	 * a monitor.
	 *
	 * @throws IllegalMonitorStateException if the current thread is not
	 *                                      the owner of this object's monitor.
	 * @see java.lang.Object#notify()
	 * @see java.lang.Object#wait()
	 */
	public final native void notifyAll();

	/**
	 * Causes the current thread to wait until either another thread invokes the
	 * {@link java.lang.Object#notify()} method or the
	 * {@link java.lang.Object#notifyAll()} method for this object, or a
	 * specified amount of time has elapsed.
	 * <p>
	 * The current thread must own this object's monitor.
	 * <p>
	 * This method causes the current thread (call it <var>T</var>) to
	 * place itself in the wait set for this object and then to relinquish
	 * any and all synchronization claims on this object. Thread <var>T</var>
	 * becomes disabled for thread scheduling purposes and lies dormant
	 * until one of four things happens:
	 * <ul>
	 * <li>Some other thread invokes the {@code notify} method for this
	 * object and thread <var>T</var> happens to be arbitrarily chosen as
	 * the thread to be awakened.
	 * <li>Some other thread invokes the {@code notifyAll} method for this
	 * object.
	 * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
	 * thread <var>T</var>.
	 * <li>The specified amount of real time has elapsed, more or less.  If
	 * {@code timeout} is zero, however, then real time is not taken into
	 * consideration and the thread simply waits until notified.
	 * </ul>
	 * The thread <var>T</var> is then removed from the wait set for this
	 * object and re-enabled for thread scheduling. It then competes in the
	 * usual manner with other threads for the right to synchronize on the
	 * object; once it has gained control of the object, all its
	 * synchronization claims on the object are restored to the status quo
	 * ante - that is, to the situation as of the time that the {@code wait}
	 * method was invoked. Thread <var>T</var> then returns from the
	 * invocation of the {@code wait} method. Thus, on return from the
	 * {@code wait} method, the synchronization state of the object and of
	 * thread {@code T} is exactly as it was when the {@code wait} method
	 * was invoked.
	 * <p>
	 * A thread can also wake up without being notified, interrupted, or
	 * timing out, a so-called <i>spurious wakeup</i>.  While this will rarely
	 * occur in practice, applications must guard against it by testing for
	 * the condition that should have caused the thread to be awakened, and
	 * continuing to wait if the condition is not satisfied.  In other words,
	 * waits should always occur in loops, like this one:
	 * <pre>
	 *     synchronized (obj) {
	 *         while (&lt;condition does not hold&gt;)
	 *             obj.wait(timeout);
	 *         ... // Perform action appropriate to condition
	 *     }
	 * </pre>
	 * (For more information on this topic, see Section 3.2.3 in Doug Lea's
	 * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
	 * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
	 * Language Guide" (Addison-Wesley, 2001).
	 *
	 * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
	 * interrupted} by any thread before or while it is waiting, then an
	 * {@code InterruptedException} is thrown.  This exception is not
	 * thrown until the lock status of this object has been restored as
	 * described above.
	 *
	 * <p>
	 * Note that the {@code wait} method, as it places the current thread
	 * into the wait set for this object, unlocks only this object; any
	 * other objects on which the current thread may be synchronized remain
	 * locked while the thread waits.
	 * <p>
	 * This method should only be called by a thread that is the owner
	 * of this object's monitor. See the {@code notify} method for a
	 * description of the ways in which a thread can become the owner of
	 * a monitor.
	 *
	 * @param timeout the maximum time to wait in milliseconds.
	 * @throws IllegalArgumentException     if the value of timeout is
	 *                                      negative.
	 * @throws IllegalMonitorStateException if the current thread is not
	 *                                      the owner of the object's monitor.
	 * @throws InterruptedException         if any thread interrupted the
	 *                                      current thread before or while the current thread
	 *                                      was waiting for a notification.  The <i>interrupted
	 *                                      status</i> of the current thread is cleared when
	 *                                      this exception is thrown.
	 * @see java.lang.Object#notify()
	 * @see java.lang.Object#notifyAll()
	 */
	public final native void wait(long timeout) throws InterruptedException;

	/**
	 * Causes the current thread to wait until another thread invokes the
	 * {@link java.lang.Object#notify()} method or the
	 * {@link java.lang.Object#notifyAll()} method for this object, or
	 * some other thread interrupts the current thread, or a certain
	 * amount of real time has elapsed.
	 * <p>
	 * This method is similar to the {@code wait} method of one
	 * argument, but it allows finer control over the amount of time to
	 * wait for a notification before giving up. The amount of real time,
	 * measured in nanoseconds, is given by:
	 * <blockquote>
	 * <pre>
	 * 1000000*timeout+nanos</pre></blockquote>
	 * <p>
	 * In all other respects, this method does the same thing as the
	 * method {@link #wait(long)} of one argument. In particular,
	 * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
	 * <p>
	 * The current thread must own this object's monitor. The thread
	 * releases ownership of this monitor and waits until either of the
	 * following two conditions has occurred:
	 * <ul>
	 * <li>Another thread notifies threads waiting on this object's monitor
	 *     to wake up either through a call to the {@code notify} method
	 *     or the {@code notifyAll} method.
	 * <li>The timeout period, specified by {@code timeout}
	 *     milliseconds plus {@code nanos} nanoseconds arguments, has
	 *     elapsed.
	 * </ul>
	 * <p>
	 * The thread then waits until it can re-obtain ownership of the
	 * monitor and resumes execution.
	 * <p>
	 * As in the one argument version, interrupts and spurious wakeups are
	 * possible, and this method should always be used in a loop:
	 * <pre>
	 *     synchronized (obj) {
	 *         while (&lt;condition does not hold&gt;)
	 *             obj.wait(timeout, nanos);
	 *         ... // Perform action appropriate to condition
	 *     }
	 * </pre>
	 * This method should only be called by a thread that is the owner
	 * of this object's monitor. See the {@code notify} method for a
	 * description of the ways in which a thread can become the owner of
	 * a monitor.
	 *
	 * @param timeout the maximum time to wait in milliseconds.
	 * @param nanos   additional time, in nanoseconds range
	 *                0-999999.
	 * @throws IllegalArgumentException     if the value of timeout is
	 *                                      negative or the value of nanos is
	 *                                      not in the range 0-999999.
	 * @throws IllegalMonitorStateException if the current thread is not
	 *                                      the owner of this object's monitor.
	 * @throws InterruptedException         if any thread interrupted the
	 *                                      current thread before or while the current thread
	 *                                      was waiting for a notification.  The <i>interrupted
	 *                                      status</i> of the current thread is cleared when
	 *                                      this exception is thrown.
	 */
	public final void wait(long timeout, int nanos) throws InterruptedException {
		if (timeout < 0) {
			throw new IllegalArgumentException("timeout value is negative");
		}

		if (nanos < 0 || nanos > 999999) {
			throw new IllegalArgumentException("nanosecond timeout value out of range");
		}

		if (nanos > 0) {
			timeout++;
		}

		wait(timeout);
	}

	/**
	 * Causes the current thread to wait until another thread invokes the
	 * {@link java.lang.Object#notify()} method or the
	 * {@link java.lang.Object#notifyAll()} method for this object.
	 * In other words, this method behaves exactly as if it simply
	 * performs the call {@code wait(0)}.
	 * <p>
	 * The current thread must own this object's monitor. The thread
	 * releases ownership of this monitor and waits until another thread
	 * notifies threads waiting on this object's monitor to wake up
	 * either through a call to the {@code notify} method or the
	 * {@code notifyAll} method. The thread then waits until it can
	 * re-obtain ownership of the monitor and resumes execution.
	 * <p>
	 * 从新判断
	 * As in the one argument version, interrupts and spurious wakeups are
	 * possible, and this method should always be used in a loop:
	 * <pre>
	 *     synchronized (obj) {
	 *         while (&lt;condition does not hold&gt;)
	 *             obj.wait();
	 *         ... // Perform action appropriate to condition
	 *     }
	 * </pre>
	 * This method should only be called by a thread that is the owner
	 * of this object's monitor. See the {@code notify} method for a
	 * description of the ways in which a thread can become the owner of
	 * a monitor.
	 *
	 * @throws IllegalMonitorStateException if the current thread is not
	 *                                      the owner of the object's monitor.
	 * @throws InterruptedException         if any thread interrupted the
	 *                                      current thread before or while the current thread
	 *                                      was waiting for a notification.  The <i>interrupted
	 *                                      status</i> of the current thread is cleared when
	 *                                      this exception is thrown.
	 * @see java.lang.Object#notify()
	 * @see java.lang.Object#notifyAll()
	 */
	public final void wait() throws InterruptedException {
		wait(0);
	}

	/**
	 * 在垃圾收集器执行的时候会调用被回收对象的此方法，可以覆盖此方法提供垃圾收集时的其他资源回收，
	 * 例如关闭文件等. JVM不保证此方法总被调用
	 * Called by the garbage collector on an object when garbage collection
	 * determines that there are no more references to the object.
	 * A subclass overrides the {@code finalize} method to dispose of
	 * system resources or to perform other cleanup.
	 * <p>
	 * The general contract of {@code finalize} is that it is invoked
	 * if and when the Java&trade; virtual
	 * machine has determined that there is no longer any
	 * means by which this object can be accessed by any thread that has
	 * not yet died, except as a result of an action taken by the
	 * finalization of some other object or class which is ready to be
	 * finalized. The {@code finalize} method may take any action, including
	 * making this object available again to other threads; the usual purpose
	 * of {@code finalize}, however, is to perform cleanup actions before
	 * the object is irrevocably discarded. For example, the finalize method
	 * for an object that represents an input/output connection might perform
	 * explicit I/O transactions to break the connection before the object is
	 * permanently discarded.
	 * <p>
	 * The {@code finalize} method of class {@code Object} performs no
	 * special action; it simply returns normally. Subclasses of
	 * {@code Object} may override this definition.
	 * <p>
	 * The Java programming language does not guarantee which thread will
	 * invoke the {@code finalize} method for any given object. It is
	 * guaranteed, however, that the thread that invokes finalize will not
	 * be holding any user-visible synchronization locks when finalize is
	 * invoked. If an uncaught exception is thrown by the finalize method,
	 * the exception is ignored and finalization of that object terminates.
	 * <p>
	 * After the {@code finalize} method has been invoked for an object, no
	 * further action is taken until the Java virtual machine has again
	 * determined that there is no longer any means by which this object can
	 * be accessed by any thread that has not yet died, including possible
	 * actions by other objects or classes which are ready to be finalized,
	 * at which point the object may be discarded.
	 * <p>
	 * The {@code finalize} method is never invoked more than once by a Java
	 * virtual machine for any given object.
	 * <p>
	 * Any exception thrown by the {@code finalize} method causes
	 * the finalization of this object to be halted, but is otherwise
	 * ignored.
	 *
	 * @throws Throwable the {@code Exception} raised by this method
	 * @jls 12.6 Finalization of Class Instances
	 * @see java.lang.ref.WeakReference
	 * @see java.lang.ref.PhantomReference
	 */
	protected void finalize() throws Throwable {
	}
}
